File:The Quarterly journal of the Geological Society of London (13937153101).jpg

266

MISS

 C. 
 A. 
 RAISIN 
 ON 
 SOME 
 

broken

 and 
 moved 
 along 
 in 
 a 
 viscous 
 state 
 by 
 a 
 renewed 
 now 
 of 
 the 
 

rock,

 and 
 thus 
 caused 
 to 
 assume 
 an 
 oval 
 form, 
 with 
 more 
 or 
 less 
 

lamination

 of 
 the 
 crust. 
 

At

 Pen-y-chain 
 I 
 found 
 in 
 situ 
 one 
 illustration 
 of 
 spheroidal 
 forma- 
 

tion

 induced 
 around 
 a 
 foreign 
 centre,where 
 the 
 nucleus 
 was 
 a 
 rounded 
 

agate-nodule,

 three 
 inches 
 across, 
 with 
 a 
 thick 
 crust 
 and 
 an 
 interior 
 

filled

 with 
 white 
 quartz. 
 Around 
 this, 
 cutting 
 across 
 the 
 lamination 
 

of

 the 
 perlitic 
 and 
 spherulitic 
 rock, 
 was 
 a 
 crack 
 defining 
 a 
 sphere 
 some 
 

13

 inches 
 in 
 diameter. 
 The 
 part 
 within 
 the 
 crack 
 was 
 slightly 
 

darkened,

 but 
 otherwise 
 similar 
 to 
 the 
 mass 
 of 
 the 
 rock. 
 Prof. 
 

Bonney

 describes, 
 in 
 a 
 microscopic 
 slide, 
 a 
 cracking 
 similarly 
 

caused

 by 
 a 
 strain 
 around 
 spherulites 
 in 
 a 
 devitrified 
 glass 
 *. 
 

I

 might 
 mention 
 also, 
 in 
 connexion 
 with 
 these 
 specimens, 
 an 
 

example

 which 
 I 
 found 
 some 
 years 
 ago 
 in 
 a 
 road-heap 
 near 
 Dolgelly. 
 

The

 rock 
 is 
 a 
 kind 
 of 
 diabase, 
 and 
 has 
 a 
 dark 
 ground-mass, 
 in 
 which 
 

are

 imbedded 
 grey, 
 flinty-looking, 
 oval- 
 shaped 
 nodules 
 without 
 

radial

 structure. 
 The 
 matrix 
 is 
 a 
 slaggy-looking 
 mass 
 with 
 deposit 
 

of

 viridite, 
 and 
 contains 
 small 
 felspars 
 of 
 plagioclase 
 form. 
 The 
 

nodules

 are 
 similar, 
 except 
 that 
 viridite 
 is 
 absent 
 from 
 their 
 devit- 
 

rified

 ground- 
 mass, 
 although 
 it 
 occasionally 
 spreads 
 along 
 cracks, 
 or 
 

aggregates

 around 
 what 
 are 
 apparently 
 central 
 cavities 
 filled 
 with 
 

quartz.

 Porphyritic 
 felspars, 
 both 
 orthoclase 
 and 
 plagioclase, 
 occur, 
 

generally

 normal 
 in 
 appearance 
 ; 
 but 
 one 
 crystal 
 seems 
 to 
 be 
 replaced 
 

by

 quartz-grains. 
 There 
 is 
 small 
 epidote, 
 occurring 
 in 
 connexion 
 with 
 

dark

 opaque 
 crystals. 
 

Summary.

I.

 At 
 both 
 the 
 headlands 
 of 
 Pen-y-chain 
 and 
 Careg-y-defaid 
 the 
 

character

 of 
 the 
 rocks 
 clearly 
 negatives 
 the 
 theory 
 of 
 intrusion. 
 

They

 are 
 old 
 lava-flows, 
 once 
 glassy, 
 now 
 devitrified, 
 and, 
 at 
 Pen-y- 
 

chain,

 with 
 interbedded 
 agglomeratic 
 and 
 ashy 
 strata. 
 The 
 pro- 
 

portion

 of 
 silica 
 has 
 probably 
 undergone 
 subsequent 
 alteration, 
 but 
 the 
 

frequency

 of 
 porphyritic 
 plagioclase 
 suggests 
 that 
 the 
 rocks 
 approach 
 

nearer

 to 
 dacites 
 than 
 to 
 rhyolites 
 f 
 . 
 As 
 to 
 the 
 slratigraphical 
 

position

 of 
 the 
 Pen-y-chain 
 rocks, 
 apart 
 from 
 any 
 vague 
 suggestion 
 

of

 lithological 
 resemblance, 
 the 
 general 
 dip 
 and 
 the 
 indications 
 of 
 a 
 

succession

 near 
 Llym 
 gwyn 
 seem 
 to 
 afford 
 a 
 tolerable 
 certainty 
 of 
 

the

 Bala 
 age 
 of 
 these 
 volcanic 
 accumulations. 
 The 
 dip 
 is 
 towards 
 

the

 north 
 or 
 east 
 of 
 north, 
 and 
 the 
 rocks 
 therefore 
 belong 
 to 
 the 
 

southern

 part 
 of 
 the 
 synclinal 
 in 
 which 
 the 
 Bala 
 beds 
 of 
 the 
 Lleyn 
 

are

 arranged. 
 As 
 suggested 
 in 
 the 
 Survey 
 memoir 
 J, 
 this 
 synclinal 
 

may

 be 
 traced 
 eastwards, 
 and 
 the 
 ashy 
 beds 
 near 
 Pwllheli 
 may 
 be 
 

taken

 to 
 represent 
 similar 
 strata 
 of 
 Snowdon 
 and 
 Moel 
 Hebog. 
 In 
 

like

 manner 
 we 
 may 
 probably 
 correlate 
 the 
 Pen-y-chain 
 rocks 
 with 
 

felsites

 exposed 
 in 
 those 
 mountains. 
 The 
 mass 
 of 
 felstone 
 also 
 near 
 

Pwllheli,

 examined 
 either 
 in 
 the 
 field 
 or 
 by 
 the 
 microscope, 
 shows 
 

 Pres. 
 Adtfr. 
 Geol. 
 Soc. 
 1885, 
 p. 
 64. 
 

t

 Cf. 
 Kosenbusch, 
 ' 
 Mikr. 
 Pkys. 
 der 
 rnassigen 
 Gesteine,' 
 ii. 
 Abth. 
 p. 
 418. 
 

+

 Geol. 
 of 
 North 
 Wales, 
 p.